The Attentional Blink Is Governed by a Temporary Loss of Control

Article excerpt

Identification of the second of two brief targets is impaired at intertarget lags of less than about 500 msec. We compared two accounts of this attentional blink (AB) by manipulating the number of digit distractors-and hence the lag-inserted among three letter targets in a rapid serial visual presentation stream of digit distractors. On the resource-depletion hypothesis, longer lags provide more time for processing the leading target, thus releasing resources for the trailing target. On the temporary-loss-of-control (TLC) hypothesis, intervening distractors disrupt the current attentional set, producing a trailing-target deficit. Identification accuracy for trailing targets was unimpaired not only at lag 1 (conventional lag 1 sparing) but also at later lags, if preceded by another target. The results supported the TLC hypothesis but not the resource-depletion hypothesis. We conclude that the AB is caused by a disruption in attentional set when a distractor is presented while the central executive is busy processing a leading target.

When two visual stimuli are presented in close temporal succession, identification is almost perfect for the first stimulus (T1) but substantially impaired for the second (T2). This second-target deficit, called the attentional blink (AB; Raymond, Shapiro, & Arnell, 1992), has been studied using a paradigm in which observers are required to identify two targets (e.g., letters) inserted in a stream of distractors (e.g., digits) displayed in rapid serial visual presentation (RSVP). All items in the RSVP stream are presented in the same spatial location at a rate of approximately 10 items/sec. The temporal lag between the two targets is manipulated by varying the number of intervening distractors. The second-target deficit is most pronounced at short intertarget lags, with performance improving progressively as the lag is increased, reaching an asymptote at about 500 msec.

Theoretical accounts of the AB have focused on the processing of T1 as the primary source of the second-target deficit. On these accounts (e.g., Chun & Potter, 1995; Jolicoeur & Dell' Acqua, 1998; Shapiro, Raymond, & Arnell, 1994), the requirement to process T1 is said to delay the allocation of attentional resources to T2 for several hundred milliseconds. As a result, if T2 is presented shortly after T1, its processing is delayed, and it becomes vulnerable to overwriting by subsequent stimuli (see, e.g., Giesbrecht & Di Lollo, 1998). At longer lags, resources initially deployed to T1 become available for T2 as T1 processing is nearing completion, and the AB is no longer in evidence. In brief, current theoretical accounts of the AB hold to the idea of resource depletion as the principal factor in the second-target deficit.

Findings inconsistent with a resource-depletion account have been reported by Di Lollo, Kawahara, Ghorashi, and Enns (2005) in a study in which observers were required to identify three consecutive target letters inserted in an RSVP stream of digit distractors. According to the resource-depletion hypothesis, identification accuracy should be highest for the leading target and decline progressively for each successive target as resources become more depleted. Instead, no progressive decrement (i.e., no AB deficit) was in evidence over the successive targets: The third target (T3) was identified as accurately as the first. A second, and even more revealing finding was that identification of T3 was substantially impaired-as in the conventional AB deficit-when the middle letter in the three-target string was replaced with a digit. This is all the more remarkable because the observers were required to report only two targets rather than three targets.

An account of these findings has been proposed by Di Lollo et al. (2005) in terms of a temporary loss of control (TLC) over the prevailing attentional set. The TLC account is based on two assumptions. First, that at the outset of the RSVP stream, the observer adopts an attentional set aimed at accepting targets and rejecting distractors. …